The fact that it cannot be guaranteed to remove all silver and halides with every film in every case. This may cause increased graininess, increased density and subdued colors. This is a summary of the discussions, the bottom line being that Kodak has studied the film blixes but decided not to use them. Use the search function to find more... PE has written most of the posts about this subject. This was discussed more frequently a few years ago.

However, in most cases, blixes seem to work well enough, or even perfectly, and if nothing else is available, why not use them. This is a typical example of differences between "99%" and "100%".

It's just funny; separate bleach and fix are easier to design, keep better, are the standard process, and are guaranteed to work. Some small manufacturers apparently try to act wiser than they actually are, and try to show off with skills they do not actually have, designing "revolutionary" products that are not that revolutionary, just problematic. The catastrophic design of Tetenal's "mono-concentrates" (RA-4) is a good example of this. This "innovation" is luckily not used in their C-41 or E-6 chemicals.